[PATCH 10/10] perf/uprobe: Add uretprobe timer

[Peter Zijlstra]

In order to put a bound on the uretprobe_srcu critical section, add a
timer to uprobe_task. Upon every RI added or removed the timer is
pushed forward to now + 1s. If the timer were ever to fire, it would
convert the SRCU 'reference' to a refcount reference if possible.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 include/linux/uprobes.h |    8 +++++
 kernel/events/uprobes.c |   66 ++++++++++++++++++++++++++++++++++++++++++++----
 2 files changed, 68 insertions(+), 6 deletions(-)

--- a/include/linux/uprobes.h
+++ b/include/linux/uprobes.h
@@ -15,6 +15,7 @@
 #include <linux/rbtree.h>
 #include <linux/types.h>
 #include <linux/wait.h>
+#include <linux/timer.h>
 
 struct vm_area_struct;
 struct mm_struct;
@@ -79,6 +80,10 @@ struct uprobe_task {
 	struct return_instance		*return_instances;
 	unsigned int			depth;
 	unsigned int			active_srcu_idx;
+
+	struct timer_list		ri_timer;
+	struct callback_head		ri_task_work;
+	struct task_struct		*task;
 };
 
 struct return_instance {
@@ -86,7 +91,8 @@ struct return_instance {
 	unsigned long		func;
 	unsigned long		stack;		/* stack pointer */
 	unsigned long		orig_ret_vaddr; /* original return address */
-	bool			chained;	/* true, if instance is nested */
+	u8			chained;	/* true, if instance is nested */
+	u8			has_ref;
 	int			srcu_idx;
 
 	struct return_instance	*next;		/* keep as stack */
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1742,7 +1742,12 @@ unsigned long uprobe_get_trap_addr(struc
 static struct return_instance *free_ret_instance(struct return_instance *ri)
 {
 	struct return_instance *next = ri->next;
-	srcu_read_unlock(&uretprobes_srcu, ri->srcu_idx);
+	if (ri->uprobe) {
+		if (ri->has_ref)
+			put_uprobe(ri->uprobe);
+		else
+			srcu_read_unlock(&uretprobes_srcu, ri->srcu_idx);
+	}
 	kfree(ri);
 	return next;
 }
@@ -1766,11 +1771,48 @@ void uprobe_free_utask(struct task_struc
 	while (ri)
 		ri = free_ret_instance(ri);
 
+	timer_delete_sync(&utask->ri_timer);
+	task_work_cancel(utask->task, &utask->ri_task_work);
 	xol_free_insn_slot(t);
 	kfree(utask);
 	t->utask = NULL;
 }
 
+static void return_instance_task_work(struct callback_head *head)
+{
+	struct uprobe_task *utask = container_of(head, struct uprobe_task, ri_task_work);
+	struct return_instance *ri;
+
+	for (ri = utask->return_instances; ri; ri = ri->next) {
+		if (!ri->uprobe)
+			continue;
+		if (ri->has_ref)
+			continue;
+		if (refcount_inc_not_zero(&ri->uprobe->ref))
+			ri->has_ref = true;
+		else
+			ri->uprobe = NULL;
+		srcu_read_unlock(&uretprobes_srcu, ri->srcu_idx);
+	}
+}
+
+static void return_instance_timer(struct timer_list *timer)
+{
+	struct uprobe_task *utask = container_of(timer, struct uprobe_task, ri_timer);
+	task_work_add(utask->task, &utask->ri_task_work, TWA_SIGNAL);
+}
+
+static struct uprobe_task *alloc_utask(struct task_struct *task)
+{
+	struct uprobe_task *utask = kzalloc(sizeof(struct uprobe_task), GFP_KERNEL);
+	if (!utask)
+		return NULL;
+	timer_setup(&utask->ri_timer, return_instance_timer, 0);
+	init_task_work(&utask->ri_task_work, return_instance_task_work);
+	utask->task = task;
+	return utask;
+}
+
 /*
  * Allocate a uprobe_task object for the task if necessary.
  * Called when the thread hits a breakpoint.
@@ -1782,7 +1824,7 @@ void uprobe_free_utask(struct task_struc
 static struct uprobe_task *get_utask(void)
 {
 	if (!current->utask)
-		current->utask = kzalloc(sizeof(struct uprobe_task), GFP_KERNEL);
+		current->utask = alloc_utask(current);
 	return current->utask;
 }
 
@@ -1791,7 +1833,7 @@ static int dup_utask(struct task_struct
 	struct uprobe_task *n_utask;
 	struct return_instance **p, *o, *n;
 
-	n_utask = kzalloc(sizeof(struct uprobe_task), GFP_KERNEL);
+	n_utask = alloc_utask(t);
 	if (!n_utask)
 		return -ENOMEM;
 	t->utask = n_utask;
@@ -1803,13 +1845,20 @@ static int dup_utask(struct task_struct
 			return -ENOMEM;
 
 		*n = *o;
-		__srcu_clone_read_lock(&uretprobes_srcu, n->srcu_idx);
+		if (n->uprobe) {
+			if (n->has_ref)
+				get_uprobe(n->uprobe);
+			else
+				__srcu_clone_read_lock(&uretprobes_srcu, n->srcu_idx);
+		}
 		n->next = NULL;
 
 		*p = n;
 		p = &n->next;
 		n_utask->depth++;
 	}
+	if (n_utask->return_instances)
+		mod_timer(&n_utask->ri_timer, jiffies + HZ);
 
 	return 0;
 }
@@ -1957,6 +2006,8 @@ static void prepare_uretprobe(struct upr
 	ri->next = utask->return_instances;
 	utask->return_instances = ri;
 
+	mod_timer(&utask->ri_timer, jiffies + HZ);
+
 	return;
 fail:
 	kfree(ri);
@@ -2151,6 +2202,9 @@ handle_uretprobe_chain(struct return_ins
 	struct uprobe *uprobe = ri->uprobe;
 	struct uprobe_consumer *uc;
 
+	if (!uprobe)
+		return;
+
 	guard(srcu)(&uprobes_srcu);
 
 	for_each_consumer_rcu(uc, uprobe->consumers) {
@@ -2197,8 +2251,10 @@ static void handle_trampoline(struct pt_
 
 		instruction_pointer_set(regs, ri->orig_ret_vaddr);
 		do {
-			if (valid)
+			if (valid) {
 				handle_uretprobe_chain(ri, regs);
+				mod_timer(&utask->ri_timer, jiffies + HZ);
+			}
 			ri = free_ret_instance(ri);
 			utask->depth--;
 		} while (ri != next);